The underlying premise of Flux Destiny is that people within the universe have discovered that they can create their own realities... or Destinies. These destinies are better known as Flux Destinies, artificial realities created not by the natural laws of cause-and-effect, but rather by carefully engineering the Gears of Destiny (the molecular level of the world).
Creating realities out of thin air is a big deal, and as you can imagine, would create all sorts of disastrous situations for all people living in that world. Worse, it could make for some pretty cruddy storytelling if anyone could just conjure their own reality anytime they thought about it. It's like the 3-wishes problem with the genie in the lamp... why don't you just wish for unlimited wishes?
That said, I constrained this awesome power by using logical boundaries... a big enough Flux Destiny could ripple and overwrite the Main Sequence - the reality of that world. The Great Machine is the mechanism for cause-and-effect in this universe, and everyone is connected to it. It's big enough to be able to absorb small Flux Destinies (like the one pictured in the drawings in this entry).
Another constraint I utilized is time. Depending on the size, most Flux Destinies collapse or ripple within 6 minutes and 46 seconds (a number that is personal to me and I'll probably never reveal why I chose it). It also allows for tension in action sequences... 6 minutes isn't a lot of time to do much, so it keeps the screws tightened on the characters.